1. Field of the Invention
The present invention relates to a blocking oscillator power pack for supplying an electric device in which the primary winding of the transformer is connected in series with a segment of a switching transistor carrying current to be switched, being connected in series therewith to a direct voltage obtained by way of rectification of the line alternating voltage supply via two external supply terminals, and in which a secondary winding of the transformer is provided for the power supply of the electric device, and further in which the control electrode of the switching transistor is controlled by the output of the control circuit which is, in turn, charged with the rectified line alternating voltage as an actual value and by a reference value generator.
2. Description of the Prior Art
Blocking oscillator power packs are disclosed, for example in the publication "Funkschau", 1975, No. 5, pp. 40-44, in the German published application No. 30 32 034, and in the German published application No. P 33 30 039.9.
As known, such a power pack has the task of supplying an electronic device, for example a television receiver, with stabilized and regulated operating voltages. The core of such a power pack is therefore provided by a control circuit whose final control element is realized by way of a switching transistor, particularly by way of a bipolar power transistor. The fundamental circuit diagram appertaining to such a power pack is illustrated in FIG. 1 and is discussed first herein.
An npn power transistor T serves as the final control element for a control circuit RS and has its emitter/collector segment connected in series with a primary winding W.sub.p of a transformer TR. With reference to FIG. 1 of the German published application No. 30 32 034, it can thereby be determined that the direct voltage operating this series connection is obtained by way of rectification of the alternating voltage supplied by the a.c. network with a rectifier circuit, for example a bridge rectifier. Given the use of an npn transistor T, the emitter of the transistor is connected to a reference potential (ground), the collector lies at the primary winding W.sub.p of the transformer Tr and the other end of the primary winding is connected to the supply potential +U.sub.p supplied by the rectifier circuit (which, however, is not shown on the drawings). The emitter-collector segment of the transistor T is bridged by a capacitor C.sub.s, whereas a capacitor C.sub.w indicated at the primary winding W.sub.p is of a parasitic nature. At its base, the power transistor T is controlled by the output portion of the control circuit RS, which is preferably represented by a pulse duration modulator PDM.
An auxiliary winding W.sub.H of the transformer Tr, which is designed as a secondary winding in the illustrated exemplary case, serves as a sensor for the control circuit RS and therefore has its one end connected to the reference potential and its other end connected to the input of the control circuit RS. A further secondary winding W.sub.s serves the purpose of charging the electrical device R.sub.L to be supplied upon mediation of a rectifier system G.sub.L which forwards the direct voltage U.sub.s to the device.
In the examples illustrated on the drawings, the control circuit RS comprises an output circuit portion PDM which controls the transistor T and is designed as a pulse duration modulator and of two input portions controlled by the auxiliary winding W.sub.H, whereby the one input portion RSE serves for generating the control voltage and emits a control signal U.sub.A for the output portion PDM via a controlled-gain amplifier RV. The other input portion IAB serves the purpose of pulse editing and supplies a signal U.sub.N to the output portion PDM of the control circuit RS. Finally, a current-voltage transformer SSW is also provided which forms the actual value control of the control circuit RS and emits a voltage U.sub.Ip to the pulse duration modulator that is proportional to the primary current I.sub.p. The last-mentioned portions of the control circuit RS are likewise set forth in the aforementioned German published application No. 30 32 034. They belong to the control circuit illustrated in FIG. 3 thereof. The control voltage generation is provided therein by the resistors R5 and R4 to be seen in FIGS. 1 and 2 thereof. The pulse editor IAB comprises a zero passage identifier and a control logic charged by the identifier that may be seen in FIG. 3 of the aforementioned German published application. The pulse duration modulator PDM, finally, is represented by the trigger circuit indicated in the German published application No. 30 32 034 together with that portion of the control logic that is charged by the trigger circuit.
The timing diagram appertaining to a circuit according to FIG. 1 of the present application, i.e. the chronological behavior of the signals appearing in the control circuit, namely the signals U.sub.H (the signal emitted by the transformer winding W.sub.H for the control of the control circuit), U.sub.N (the signal supplied by the pulse editor IAB), I.sub.p (the current supplied by the transformer winding W.sub.p which is connected in series with the switching transistor T), and U.sub.Ip (the actual value signal supplied by the current-voltage transformer SSW) is shown in FIG. 2.
As may be seen, the voltage U.sub.H having the zero passage (U.sub.H =0V) which is supplied by the transformer winding W.sub.H supplies the information that the energy stored in the transformer Tr has flowed off and a new charging cycle can begin, i.e. the switch given by the transistor T can be closed. This information is communicated to the pulse duration modulator PDM via the pulse editing stage IAB. (Is therefore true that U.sub.N &gt;0 volts ' pulse start, U.sub.N &lt;0 volts .fwdarw. no pulse start possible).
A control voltage U.sub.R, which is proportional to the secondary voltage U.sub.s, is also acquired with the assistance of the control voltage generator RSE from the signal voltage U.sub.H supplied by the winding W.sub.H of the transformer Tr. The control voltage is compared to a reference in the control-gain amplifier RV. The difference between the control voltage U.sub.R and the reference is amplified by the controlled-gain amplifier RV and the signal voltage U.sub.A supplied by the output thereof is communicated to the pulse duration modulator PDM which compares it to the signal U.sub.Ip of the current-voltage transformer SSW and opens the switch represented by the transistor T as soon as U.sub.Ip =U.sub.A is valid. The peak value I.sub.pmax of the current I.sub.p is corrected in this manner until the difference between the voltage U.sub.R and the reference voltage disappears. This means that the voltage U.sub.R and, therefore, the voltage U.sub.s remain constant.
Given blocking oscillator power pack of the type just described, but also generally given was of the type initially defined, it has been observed that, given certain disruption cases at the primary side of the transformer Tr, the power packs have a tendency to lead to a great super-elevation of the secondary voltages which can, under given conditions, lead to destruction in the load circuit charged by the power pack. Such disruption can be caused, for example, due to defects in the components forming the power pack, due to transgression of tolerances, due to contact interruptions, etc.